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Logitech Z5500 diagnosis help

Discussion in 'Troubleshooting and Repair' started by Mutiny, Mar 6, 2014.

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  1. Mutiny

    Mutiny

    6
    0
    Mar 6, 2014
    First off, I'm clearly new here, so hello to everyone.

    My reason for joining is to hopefully find someone who may be able to offer assistance in diagnosing what is wrong with my Logitech speakers.

    Problem history: (it's a bit lengthy but please bear with me).

    I'll start with the first anomaly with these speakers. A couple of years ago I thought they had died when they suddenly lost all power, checked the plug fuse and the slow blow fuse in the back of the sub and they were both good but still no light on the control unit as if a fuse had gone.

    Left them unplugged over night and the next morning when I tried them they powered up and worked perfectly.

    Same thing happened again after many months, and the same thing fixed it.

    Recently though, they died and now completely refuse to power up.



    Easy solution: bought another set on eBay.

    They are not the same revision but they are fully functional. Job done.



    Difficult solution, but far more satisfying:

    I have had my logitech speakers for about 13 years and have grown quite sentimental about them and would love to get them repaired. I don't think there is much wrong with them since when they did power up they worked perfectly.


    Progress so far:

    I have done a visual check for bulging capacitors but they all look fine, I have opened up the working set of speakers to compare voltages etc. but the circuitry has a completely different layout. Despite this I have managed to find some correlations between the two and believe I may have found a discrepancy with the faulty set.

    The working set has an 8v supply going to the control unit that I believe is the standby power that illuminates the power button etc. But on the faulty set this same pin is only showing about 0.2v.

    I have tried to trace this back but can only follow it to a medium-sized capacitor and adiode located on the main board.

    Could a faulty capacitor cause the issues that I have experienced?

    How can I test if a capacitor is good or bad?



    tl;dr

    A couple of years ago I thought they had died when they suddenly lost all power, checked the plug fuse and the slow blow fuse in the back of the sub and they were both good but still no light on the control unit as if a fuse had gone.

    Left them unplugged over night and the next morning when I tried them they powered up and worked perfectly.

    Could a faulty capacitor cause the issues that I have experienced?

    How can I test if a capacitor is good or bad?


    I do have pictures and can post links if it would help.
     
  2. shrtrnd

    shrtrnd

    3,743
    482
    Jan 15, 2010
    You've already got it apart, and capacitors are cheap. Just replace the questionable ones.
    My kid used Logitech speakers years ago. While they sound good, they had their
    finicky attributes.
    I'd replace the caps, and if it doesn't fix the problem, start looking at the semi-conductors.
    (Your diode and any IC that might be there).
    Really, it sould not be too expensive to fix the speakers even if you had to replace
    every discrete part in them, there just isn't much there.
    Good Luck.
     
  3. Mutiny

    Mutiny

    6
    0
    Mar 6, 2014
    Thanks for the good advice, but none of the capacitors "look" questionable.

    I found this page on testing capacitors

    It won't take long to run a few of these tests on the capacitors. It might take a while to get at them though.

    Any recommendations on where to buy components from? The last couple of caps I bought off eBay.
     
  4. davenn

    davenn Moderator

    13,596
    1,875
    Sep 5, 2009
    caps rarely go short circuit --- you need a proper capacitance meter to tell if the capacitance has changed considerably from their marked value

    Dave
     
  5. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
    Yes, you could be on the right track with the missing 8V supply. You need to find where it comes from and why it isn't there.

    Can you upload a few photos so we know what we're dealing with?
     
  6. Mutiny

    Mutiny

    6
    0
    Mar 6, 2014
    Here's a pic of the board.

    [​IMG]

    I have highlighted the contact that I believe should be +8v.



    This is the full board

    [​IMG]

    I was given the name of a local guy today who may be able to assist me in diagnosing the problem. I'll give him a call tomorrow and see if he's able to help.

    Oh, and this is a pic from the working set of speakers, it's still a set of Z5500's but they're a more recent version

    [​IMG]

    You can see the medium-sized cap and the diode are still in approximately the same location, but the board has a completely different layout.
     
    Last edited: Mar 7, 2014
  7. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
    Thanks for the pics.

    How does power get to the board? Is there a mains transformer in there somewhere? Or a separate power supply board? Or is it powered from an external power supply?
     
  8. Mutiny

    Mutiny

    6
    0
    Mar 6, 2014
    There is a toroidal transformer in the sub, 4 wires come from the transformer and go into two rectifiers. There are also 2 voltage regulators on the main board that I have tested... One is outputting 18v and the other 21v. I'm not sure if it matters but on the working set both regulators output 18v, could be something but could be nothing.

    So power is definitely getting to the main board, I just need to try and locate where the 8v is supposed to come from.
     
  9. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
    The regulator that's outputting 21V may be doing that because there's no load on it. (Or of course, it could be damaged, or it might be supposed to put out 21V because the two versions could be different.)

    Is it possible that there's a break in the feed from that regulator to another part of the circuit that produces the 8V rail? There might be a fuse in there... That could explain why the 8V rail is missing and the regulator output voltage is higher than it should be (assuming that it is...)

    Are you sure you're measuring the outputs of the regulators? Are they simple 3-terminal types?

    What are the AC voltages from the toroidal transformer going into the bridge rectifiers?
     
  10. Mutiny

    Mutiny

    6
    0
    Mar 6, 2014
    The regulators are just simple 3 pin regs.

    One of the rectifiers measures 57v ac and I didn't get anything from the 2nd rectifier. I couldn't compare this with the working sub as there is only on rectifier (of this type) in it, however the 2nd pair of wires from the transformer go to the black box on the main board, just to the left of the highlighted circle in this pic.

    [​IMG]

    I will put the black box on my to test list. I suspect this may just be an integrated rectifier, but I should be able to get a reading to compare with the duff sub.
     
  11. Mutiny

    Mutiny

    6
    0
    Mar 6, 2014
    I have identified the faulty component. It is in the toroidal transformer, there must be a dry joint or something.

    The transformer has a 230v primary with secondary of dual 26.4v (which is wired up to the rectifier as 57v) and a 3rd winding of 15v.

    It was the 15v supply that wasn't producing anything. Until I unbolted the transformer and prised it off the glue holding it to the bottom of the sub. tested it again and it was working.

    Obviously a dodgy connection somewhere in that loop so I would like to source a replacement transformer, any idea where the best place to try might be? I had a quick look on RS components but they were HUGELY expensive.
     
  12. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    8,393
    1,268
    Nov 28, 2011
    I expect it would be custom made. If you don't feel like unwinding and rewinding it, you could add another small transformer to provide the 15V AC. There's probably not much load on it.

    You won't get an accurate reading if you connect an AC ammeter in the circuit between the 15V AC secondary and where it connects to the board though, because the current drawn by the input rectifier is only in a short burst on every half-cycle (assuming it's a bridge rectifier). You need to find the rectifier and smoothing capacitor, cut the track downstream of the capacitor, and insert a DC ammeter. That will give you an accurate figure of the amount of current drawn from the winding, so that you can choose a suitable small transformer to add to the unit to provide the 15V AC.
     
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